[comment {-*- tcl -*- doctools manpage}]
[manpage_begin transfer::copy::queue n 0.1]
[keywords channel]
[keywords copy]
[keywords queue]
[keywords transfer]
[copyright {2006 Andreas Kupries <andreas_kupries@users.sourceforge.net>}]
[moddesc   {Data transfer facilities}]
[titledesc {Queued transfers}]
[category  {Transfer module}]
[require Tcl                   8.4]
[require snit                  [opt 1.0]]
[require struct::queue         [opt 1.4]]
[require transfer::copy        [opt 0.2]]
[require transfer::copy::queue [opt 0.1]]
[description]
[para]

This package provides objects which serialize transfer requests for a
single channel by means of a fifo queue. Accumulated requests are
executed in order of entrance, with the first request reaching an idle
object starting the execution in general. New requests can be added
while the object is active and are defered until all requests entered
before them have been completed successfully.

[para]

When a request causes a transfer error execution stops and all
requests coming after it are not served. Currently this means that
their completion callbacks are never triggered at all.

[para]
[emph NOTE]:

	Not triggering the completion callbacks of the unserved
	requests after an error stops the queue object is something I
	am not fully sure that it makes sense. It forces the user of
	the queue to remember the callbacks as well and run
	them. Because otherwise everything in the system which depends
	on getting a notification about the status of a request will
	hang in the air. I am slowly convincing myself that it is more
	sensible to trigger the relevant completion callbacks with an
	error message about the queue abort, and 0 bytes transfered.

[para]

All transfer requests are of the form

[para][example {
	{type data options...}
}][para]

where [arg type] is in {[const chan], [const string]}, and [arg data]
specifies the information to transfer.

For [const chan] the data is the handle of the channel containing the
actual information to transfer, whereas for [const string] [arg data]
contains directly the information to transfer.

The [arg options] are a list of them and their values, and are the
same as are accepted by the low-level copy operations of the package
[package transfer::copy].

Note how just prepending the request with [cmd transfer::copy::do] and
inserting a channel handle in between [arg data] and [arg options]
easily transforms it from a pure data structure into a command whose
evaluation will perform the request.

[section API]
[subsection {Package commands}]

[list_begin definitions]

[call [cmd transfer::copy::queue] \
	[arg objectName] \
	[arg outchannel] \
	[opt [arg options]...]]

This command creates a new queue object for the management of the
channel [arg outchannel], with an associated Tcl command whose name is
[arg objectName].

This [term object] command is explained in full detail in the sections
[sectref {Object command}] and [sectref {Object methods}]. The set of
supported [arg options] is explained in section [sectref {Options}].

[para]

The object command will be created under the current namespace if the
[arg objectName] is not fully qualified, and in the specified
namespace otherwise.

The fully qualified name of the object command is returned as the
result of the command.

[list_end]

[subsection {Object command}]

All objects created by the [cmd ::transfer::copy::queue] command have
the following general form:

[list_begin definitions]

[call [arg objectName] [method method] [opt [arg "arg arg ..."]]]

The method [method method] and its [arg arg]'uments determine the
exact behavior of the command.

See section [sectref {Object methods}] for the detailed
specifications.

[list_end]

[subsection {Object methods}]

[list_begin definitions]

[call [arg objectName] [method destroy]]

This method destroys the object. Doing so while the object is busy
will cause errors later on, when the currently executed request
completes and tries to access the now missing data structures of the
destroyed object.

[call [arg objectName] [method busy]]

This method returns a boolean value telling us if the object is
currently serving a request (i.e. [term busy], value [const True]), or
not (i.e. [term idle], value [const False]).

[call [arg objectName] [method pending]]

This method returns the number of requests currently waiting in the
queue for their execution. A request currently served is not counted
as waiting.

[call [arg objectName] [method put] [arg request]]

This method enters the transfer [arg request] into the object's queue
of waiting requests.

If the object is [term idle] it will become [term busy], immediately
servicing the request. Otherwise servicing the new request will be
defered until all preceding requests have been served.

[list_end]

[section Options]

The only option known is [option -on-status-change]. It is optional
and defaults to the empty list, disabling the reporting of status
changes. Otherwise its argument is a command prefix which is invoked
whenever the internal status of the object changed. The callback is
invoked with two additional arguments, the result of the methods
[method pending] and [method busy], in this order. This allows any
user to easily know, for example, when the object has processed all
outstanding requests.

[section Use]

A possible application of this package and class is within a HTTP 1.1
server, managing the results waiting for transfer to the client.

[para]

It should be noted that in this application the system also needs an
additional data structure which keeps track of outstanding results as
they may come back in a different order than the requests from the
client, and releases them to the actual queue in the proper order.

[vset CATEGORY transfer]
[include ../common-text/feedback.inc]
[manpage_end]
